Fuel cell system

ABSTRACT

A method of starting operation of a fuel cell system which includes at least a fuel cell stack the method includes opening an anode inlet valve to allow fuel to enter an anode volume of the fuel cell stack; then operating an air compressor in fluid communication with a cathode air inlet of the fuel cell stack to allow air to enter a cathode volume of the fuel cell stack monitoring the temperature of the cathode inlet and/or outlet operating a water injection system to inject water into the cathode volume once the temperature of fluid passing through the cathode inlet and/or outlet exceeds a preset level, wherein a current drawn from the fuel cell stack is limited to prevent a voltage measured across one or more cells in the fuel cell stack from falling below a first voltage threshold.

This application is a Divisional of patent application Ser. No.12/680,507, filed May 24, 2010, which is a National Stage filing under35 U.S.C. 371 of International Application PCT/GB2008/003225 filed on 23Sep. 2008, which claims priority from GB Application No. 0718763.6,filed 26 Sep. 2007, the entirety of each being incorporated herein byreference, as if fully set forth herein.

FIELD OF THE DISCLOSURE

The disclosure relates to the operation of, and apparatus relating to, afuel cell system, and in particular though not exclusively to a strategyfor starting operation of a fuel cell system.

BACKGROUND

Water is integral to the operation of a fuel cell system, for example inthe form of the system described herein comprising a fuel cell stackbased around a proton exchange membrane (PEM). Reaction of protons(hydrogen ions) conducted through the PEM from an anode flow path, withoxygen present in a cathode flow path, produces water. Excess waterneeds to be removed from the fuel cell stack to avoid flooding andcausing a consequent deterioration in performance. An amount of water,however, needs to be present in at least the cathode flow path tomaintain hydration of the PEM, so as to achieve optimum performance ofthe fuel cell. Managing this water, by deliberate injection and removal,can also provide a useful mechanism for removing excess heat from thefuel cell stack.

To optimize performance, water can be employed deliberately in such fuelcell systems through injection into the cathode flow path of the stack.Such water injection fuel cell systems have potential advantages ofreduced size and complexity, as compared with other types of fuel cellsystems employing separate cooling channels. Water may be injecteddirectly into the cathode flow path through water distributionmanifolds, as for example described in GB2409763.

For water injection systems, it is important that any water fed backinto the cathode flow path is of high purity, so as to avoidcontamination of the PEM and consequent degradation of stackperformance. This requirement for high purity, however, means thatadditives to lower the freezing point of water cannot be used. Forautomotive applications in particular, typical requirements includestarting up from below freezing, typically as low as −20° C. toreplicate environments in which the fuel cell may be used in practice.Since high purity water has a freezing point of 0° C. (at 1 barpressure), any water left in the fuel cell system will, given sufficienttime, freeze after shut-down of the fuel cell.

Ice in the fuel cell system, and in particular within the cathode flowpath, can prevent the stack from operating properly, or even at all. Ifany part of the cathode flow path is blocked with ice, air cannot bepassed through the cathode and the fuel cell may not be capable ofself-heating to above freezing point. Other methods of heating the wholestack will then be necessary, which will require consumption of externalpower before the fuel cell can begin supplying electrical power and heatby itself.

SUMMARY

In a first aspect, the disclosure provides a method of startingoperation of a fuel cell system comprising a fuel cell stack, the methodcomprising the steps of:

i) opening an anode inlet valve to allow fuel to enter an anode volumeof the fuel cell stack;

ii) operating an air compressor in fluid communication with a cathodeair inlet of the fuel cell stack to allow air to enter a cathode volumeof the fuel cell stack;

iii) monitoring the temperature of the cathode inlet and/or outlet; and

iv) operating a water injection system to inject water into the cathodevolume once the temperature of fluid passing through the cathode inletand/or outlet exceeds a preset level,

wherein a current drawn from the fuel cell stack is limited to prevent avoltage measured across one or more cells in the fuel cell stack fromfalling below a first voltage threshold.

In a second aspect, the disclosure provides a fuel cell stack comprisinga plurality of fuel cells, each end of the fuel cell stack having aheater plate disposed between a current collector plate and an endplate, each heater plate being thermally insulated from a respective endplate.

In a third aspect, the disclosure provides a fuel cell system comprisinga fuel cell stack and an electrical control unit configured to:

i) open an anode inlet valve to allow fuel to enter an anode volume ofthe fuel cell stack;

ii) operate an air compressor in fluid communication with a cathode airinlet of the fuel cell stack to allow air to enter a cathode volume ofthe fuel cell stack;

iii) monitor the temperature of the cathode inlet and/or outlet; and

iv) operate a water injection system to inject water into the cathodevolume once the temperature of fluid passing through the cathode inletand/or outlet exceeds a preset level,

wherein the electrical control unit is configured to limit a currentdrawn from the fuel cell stack to prevent a voltage measured across oneor more cells in the fuel cell stack from falling below a first voltagethreshold.

In a fourth aspect, the disclosure provides a fuel cell systemcomprising a fuel cell stack and an electrical control unit configuredto adjust operating parameters of the fuel cell stack to optimizeoperation of the fuel cell system based on a standard deviation ofvoltage outputs from a plurality of cells in the fuel cell stack.

In a fifth aspect, the disclosure provides a method of optimizingoperation of a fuel cell system comprising a fuel cell stack and anelectrical control unit, the method comprising:

providing an indication of a voltage output from each of a plurality ofcells in the fuel cell stack to the electrical control unit; and

optimizing operation of the fuel cell system based on a standarddeviation of the voltage outputs from the plurality of cells,

wherein the electrical control unit adjusts the operating parameters ofthe fuel cell stack to optimize operation of the fuel cell system.

Other features and advantages of the present disclosure will be setforth, in part, in the descriptions which follow and the accompanyingdrawings, wherein the implementations of the present disclosure aredescribed and shown, and in part, will become apparent to those skilledin the art upon examination of the following description taken inconjunction with the accompanying drawings or may be learned by practiceof the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will now be described by way of example only, withreference to the appended drawings in which:

FIG. 1 illustrates a schematic diagram of the arrangement of variouscomponents within an overall fuel cell system;

FIG. 2 illustrates a schematic diagram of an exemplary electricalcontrol system for a fuel cell system;

FIG. 3 illustrates a schematic side view of an exemplary fuel cellstack;

FIGS. 4 a and 4 b illustrate perspective views of an exemplary heaterplate for a fuel cell stack;

FIG. 5 illustrates a partial schematic cross-sectional view of anexemplary fuel cell stack;

FIG. 6 illustrates a schematic flow diagram of an exemplary start-upprocedure; and

FIG. 7 illustrates a series of curves showing various measuredparameters from a fuel cell system.

DETAILED DESCRIPTION

In the following description, various exemplary implementations, aspectsand characteristics are discussed as directed toward surgicalinstruments, tools, systems and methods more particularly applied to thespine. The focus on this application is not intended to be, nor shouldit act as, a limitation to the scope of this disclosure. The otherfeatures and advantages of the present disclosure will be set forth, inpart, in the descriptions which follow and the accompanying drawings,wherein the implementations of the present disclosure are described andshown, and in part, will become apparent to those skilled in the artupon examination of the following description taken in conjunction withthe accompanying drawings or may be learned by practice of the presentdisclosure.

Heading and Titles are not intended to be limitations and should be readin a general sense. Implementations may include several novel features,no single one of which is solely responsible for its desirableattributes or which is essential to practicing the disclosure describedherein. The advantages of the present disclosure may be attained bymeans of the instrumentalities and combinations particularly pointed outin the disclosure and any appended claims.

FIG. 1 shows a schematic diagram of an exemplary fuel cell system 100comprising a fuel cell stack 110 and other associated components. Thefuel cell stack 110 has a cathode flow path passing through it, thecathode flow path comprising an air inlet 124 leading to an air inletline 123 and into the stack at the cathode air inlet 126. After passingthrough an internal cathode volume (not shown) within the fuel cellstack 110, the cathode flow path exits the fuel cell stack 110 into thecathode exit line 121, through the cathode exhaust line 122 and anexhaust shutoff valve 120. During normal operation, the exhaust shut-offvalve 120 is partially or fully open. Various components such as a heatexchanger 130, with associated cooling fan 139, and a water separator131 may be connected to or part of the cathode exit line 121 and exhaustline 122 in the cathode flow path. Temperature sensors TX1, TX2, TX3,TX5 and pressure sensors PX2, PX3 may also be present, connected atappropriate places to monitor the inlet line 123 and exit line 121 ofthe cathode flow path.

The expression ‘cathode system’ in the present context is intended toencompass those parts of the fuel cell system 100 that are associatedwith the cathode volume within the fuel cell stack. These include thevarious internal components of the fuel cell such as the inlets,outlets, the internal flow path and water distribution structures, aswell as components in fluid communication with the cathode volume suchas the various inlet, outlet, recirculation and exhaust lines for bothliquids and gases. The term ‘cathode flow path’ is intended to encompassa subset of the cathode system that includes a fluid flow path from theair inlet 124 through an air compressor 133, the inlet line 123, thecathode volume of the fuel cell stack 110, and the cathode exit line121. The terms ‘anode system’ and ‘anode flow path’ are to beinterpreted similarly, with reference to the various components of thefuel cell system 100 associated with the anode volume.

The air compressor 133, connected to the cathode air inlet line 123,provides compressed air to the cathode flow path. Other components suchas an air inlet heat exchanger 134, a flow meter 135, one or more airfilters 136, 137 and an air heater 138 may be present in the cathodeinlet line 123 between the air inlet 124 and the fuel cell stack 110.The air inlet heat exchanger 134 may be used in conjunction with acoolant line 141, a three-way valve 142 and a temperature sensor TX7 topre-heat air from the air compressor 133 with coolant from the coolantline 141 during operation of the fuel cell system 100. The coolant line141 passing through the air inlet heat exchanger 134 forms a separatecooling circuit configured to extract heat from the air stream after thecompressor 133. This coolant line 141 is preferably operated after thefuel cell stack 110 reaches a normal operating temperature, in order toavoid extracting heat from the air inlet stream in the cathode air inletline 123 during start-up of the system 100. Diversion of coolant in theline 14 I may be achieved through use of the valve 142, allowing controlover whether coolant is delivered to the heat exchanger 134. Since thecoolant line 141 is separate from water fed into the cathode system, therequirement for high purity water is not the same. The coolant used inthe coolant line 141 may therefore comprise additives such as glycol tolower the freezing point of the coolant used.

Fuel, typically in the form of gaseous hydrogen, enters the fuel cellsystem via a pressure-reducing valve 151 and an actuated valve 152,preferably in the form of a normally closed solenoid-actuated valve. Thefuel supply 150, when in the form of hydrogen gas, is typically locatedremotely from the fuel cell system, for example in the form of apressurized tank towards the rear of a vehicle. A furthersolenoid-actuated valve 153 and a pressure-reducing valve 154 may beprovided closer to the fuel cell stack 110 in the fuel inlet line 155 ofthe anode flow path between the fuel source 150 and the anode inlet 156of the fuel cell stack 110. Two separate sets of valves are thereforeprovided leading to the anode inlet 156, one set 151, 152 near to thetank and the other set 153, 154 closer to the fuel cell stack 110, withan intermediate pressurized fuel line 119 in between. Thepressure-reducing valve 154 regulates the pressure of the dry fuel gasto a level suitable for introduction to the fuel cell stack 110. Thepressure-reducing valve 154 is preferably a passive device which has apreset pressure setting applied, although an actively controlled devicemay be used. A fuel heater 145 is optionally provided, for example inthe pressurized fuel line 119 before the valve 153, as shown in FIG. 1,or alternatively in the fuel inlet line 155 either before or after thepressure-reducing valve 154.

A further actuated valve 161 is provided on the anode exit line 165.Each actuated valve 152, 153, 161 may be provided with a local heaterelement to defrost the valve as required, although activation of thevalves 152, 153, 161 through passage of current through the solenoidwill provide a certain degree of heating. Preferably each of theactuated valves 152, 153, 161 is configured to be fail-safe, i.e. willonly open when actuated by current passing though the solenoid.

To monitor and to relieve pressure of fuel within the anode flow path, apressure sensor PX1 and/or pressure relief valve 157 may be provided.The pressure relief valve 157 is preferably set to open and exhaustfluid from the anode flow path through a pressure relief exhaust line158 when the pressure in the anode flow path exceeds a safe operatinglevel.

A further manually operable valve 162 in the anode exit line 165 may bepresent, this valve 162 being for used for example during servicing toensure depressurization of the anode flow path. Water build-up in theanode flow path in the fuel cell stack 110 may occur, for example as aresult of diffusion of water through the PEM from the cathode side.Consequently, an anode exhaust water separator 163 may be provided inthe anode exhaust line 164 to separate any water present in the exhaustline 164. This water can be exhausted or optionally recirculated. Duringoperation of the fuel cell stack 110, the valve 161 is typically heldclosed, and only opened intermittently to exhaust any built-up waterfrom the anode fluid path.

A cathode water injection inlet 127 is provided in the fuel cell stack110, the inlet 127 connected to a cathode water injection line 125. Thecathode water injection line 125 may be heated along a part or the wholeof its length, and extends between a water containment vessel 140 andthe cathode water injection inlet 127. A heater 129 may be provided toapply heat to a specific region of the line 125 to heat water passingthrough the injection line 125 towards the cathode water injection inlet127. A further pressure sensor PX4 may be provided on the cathode waterinjection line 125 in order to monitor the back-pressure on the line 125during operation.

Water from the cathode exit line 121 is pumped with a water pump 132,optionally provided with a heater 143, through a water return line 128towards a water containment vessel 140. Excess water is ejected from thefuel cell system 100 out of the water containment vessel 140 through awater overflow line 144.

The anode exit solenoid valve 161 is configured to regulate a saturatedgas and liquid stream exhausted from the fuel cell stack 110. As withthe anode inlet solenoid valve 153, the anode exit solenoid valve 161 iselectronically controlled and may be either open or closed, beingpreferably closed when de-energized. As the valve 161 is subjected to aliquid/saturated gas stream, water droplets may be present around thevalve when the system 100 is shut down. If the system is then subjectedto sub-zero ambient conditions, the valve 161 may then be frozen shut.Simply energizing the valve is usually insufficient to break the ice;hence a combination of external heating through a heater element 166together with use of internal heating due to the energized coil may berequired.

The heater 166 is preferably configured to apply heat to the anode exitsolenoid valve 161 as well as an anode exit water separator 163. Theheater 166 may comprise a positive temperature coefficient (PTC) heatingelement, regulated to a suitable temperature range. The anode exit linewater separator 163 is configured to separate water from the mixed gasand liquid exhaust stream from the anode exit 159 of the fuel cell stack110. Preferably, the anode exit line water separator 163 is configuredsuch that water passing through the anode water exhaust line 167 doesnot contain saturated gas in the form of bubbles in the exhaust water,so as to minimize the risk of a potentially explosive mixture arising inthe anode water exhaust. The remaining fuel gas may be recycled backinto the anode inlet 156.

The configuration of the anode system shown in FIG. 1 may also be usedto detect leaks in the fuel cell stack 110. Opening the anode inletsolenoid valve 153 while maintaining the anode exit solenoid valve 161and bypass valve 165 closed, an amount of gas is allowed to pass intothe anode volume of the fuel cell stack 110. The anode inlet solenoidvalve 153 is then closed, and the pressure at the anode inlet 156monitored over time by means of the pressure sensor PX1. Comparing thepressure as a function of time with a pre-calibrated curve, whichaccounts for loss of fuel by conduction of protons through the PEM,allows for diagnosis of any additional losses present as a result ofleaks in the fuel cell stack or in associated components in the anodeflow path.

As heat is generated whilst the valve is energized, the control strategyemployed preferably takes this into account during sub-zero operation.Although it might be unrealistic to assume that the valve can be openedimmediately when the system is started at sub-zero conditions, the timerequired to open the valve should nevertheless be minimized the pressuretransducer PX1 on the anode inlet line 155 can be used to monitoropening and closing of the anode exit valve 161, and an operationalstrategy can be changed from internal warming to normal operationaccordingly once the transducer PX1 indicates that the valve 161 isoperating correctly. Because the anode exit valve 161 is normallyclosed, the pressure transducer will register a reduction in pressure ifthe valve 161 is opened. If the valve is prevented from opening becauseof a build-up of ice, this can be registered by the absence of apressure drop on energizing the valve 161. The control strategy canconsequently be configured to apply further heating to the valve 161until a pressure drop is registered on energizing the valve 161.

To relieve pressure of fuel within the anode flow path, a pressurerelief valve 157 may be provided. The pressure relief valve 157 ispreferably set to open and exhaust fluid from the anode flow paththrough a pressure relief exhaust line 158 when the pressure in theanode flow path exceeds a safe operating level. The safe operating levelmay be set off-line using a calibrated pressure transducer and accordingto the rated pressure of the fuel cell stack 110.

A further manually operable valve 162 in the anode exit line 165 may bepresent, this valve 162 being used for example during servicing toensure depressurization of the anode flow path. Water build-up in theanode flow path in the fuel cell stack 110 may occur, for example as aresult of diffusion of water through the PEM from the cathode side.Consequently, the anode exhaust water separator 163 may be provided inthe anode exhaust line 164 to separate any water present in the exhaustline 164. This water can be exhausted or optionally recirculated. Duringoperation of the fuel cell stack 110, the valve 161 is typically heldclosed, and only opened intermittently to exhaust any built-up waterfrom the anode fluid path.

A cathode water injection inlet 127 is provided in the fuel cell stack110, the inlet 127 connected to a cathode water injection line 125. Thecathode water injection line 125 may be heated along a part or the wholeof its length, and extends between a water containment vessel 140 andthe cathode water injection inlet 127. A heater 129 may be provided toapply heat to a specific region of the line 125 to heat water passingthrough the injection line 125 towards the cathode water injection inlet127. A further pressure sensor PX4 may be provided on the cathode waterinjection line 125 in order to monitor the back-pressure on the line 125during operation.

Water from the cathode exit line 121 is pumped with a water pump 132,optionally provided with a heater 143, through a water return line 128towards the water containment vessel 140. Excess water is ejected fromthe fuel cell system 100 out of the water containment vessel 140 througha water overflow line 144. Further details of the water containmentvessel are provided in the co-pending GB application “Fuel cell system”,having the same filing date as the present application.

FIG. 2 illustrates a schematic diagram of an exemplary electricalcontrol system 200 associated with the fuel cell stack 110 of FIG. 1.Electrical power outputs 201, 202 are connected to an electrical load260, which represents various components of an automotive systemincluding a motor together with other electrically powered components.As the current applied to through the load 260 increases duringstart-up, current derived from an external power source (e.g. a batteryor, in a stationary application, mains-derived electric power) can becorrespondingly reduced to maintain the current demanded by the load260. Voltages of individual cells within the fuel cell stack 110 areoutput from the stack 110 via electrical connections to each bipolarplate, the voltages output on a plurality of voltage lines 220.Indications of the voltage from each cell are input, via a multiplexer205, to a microcontroller 210.

The voltage output of each cell of the fuel cell stack 110 can bemeasured via connection to a side tab incorporated into the design ofthe individual fuel cell bipolar plates. The side tab could be in theform of a male connector, thus allowing the use of a female push fitconnector, for example of the spade-type connectors commonly used inautomotive applications. This connection style is suitable for highlevels of vibration. The voltage of each cell may be determined withrespect to a defined zero through use of a series of differentialamplifiers in the multiplexer 205. The multiplexed voltage indicationsare input to the microcontroller 210.

The microcontroller 210 is configured to assess the voltage of each cellin the fuel cell stack 110, and to control action of two digital relaysconfigured to drive two output lines 211, 212. The digital relays, whichmay be integrated within the microcontroller 210, are controlled toindicate whether the voltage of one or more cells in the fuel cell stack110, as provided on voltage lines 220, falls below certain set thresholdvoltage values. For fail-safe purposes (for example in the case of afaulty connection), the microcontroller 210 is configured to set each ofthe output lines high only if a respective voltage threshold level isexceeded by all of the cells in the fuel cell stack. Both lines 211, 212being held high therefore indicate a ‘healthy’ state of operation of thestack. The microcontroller 210 is configured to set the digital relaysto trigger at different voltage threshold levels: a first voltagethreshold indicating a failure condition and a second voltage thresholdindicating a warning condition. Typically, the second voltage thresholdis higher than the first voltage threshold. These voltage thresholdvalues could be set via a software interface to the microcontroller 210.Thus, the digital information on output lines 211, 212, corresponding tothe first and second voltage threshold values respectively, can be usedby the fuel cell electrical control system 200 to regulate electricalcurrent drawn through the output connections 201, 202 and to adjustparameters such as air flow rate in order to actively improve the healthand durability of the fuel cell stack 110. Typical values for the firstand second voltage levels are around 0.4V and 0.6V respectively, butthese values may vary depending on various factors including the thermalbalance and acceptable load on the stack.

The use of cell voltage threshold information is a useful way ofensuring safe operation of a fuel cell, because a number of differentrecoverable faults can typically be indicated by one or more cellshaving a low voltage output. Preferably, the voltage level of the worstperforming cell is used to determine the levels set on the output lines211, 212.

In the event of a warning condition, indicated by the output line 212being set low, the control parameters of the fuel cell stack 110 can begradually adjusted, or the fuel cell current load limited, until thewarning ceases, indicated by the output line 212 being set high. In thecase of a failure condition, indicated by output line 211 being set low,the load 260 may be temporarily disconnected from the fuel cell stack110, for example by releasing an electrical contactor (not shown)installed between the fuel cell stack 110 and the electrical load 260.The load 260 can be subsequently re-connected once the failure conditionhas been removed, indicated by the microcontroller setting the outputline 211 high.

The microcontroller 210 may be replaced with hardware comparators todetermine whether cell voltages are below preset threshold levels. Alevel of software is therefore removed, thus increasing the robustnessof the technique. This may be particularly advantageous when consideringcertification and speed of response of the overall system 200.

In addition to the action of the digital relays, the microcontroller 210can also be configured to publish the cell voltage data information on aCAN (Controller Area Network) 240. The CAN allows a profile of fuel cellstack 110 voltages from voltage lines 220 to be monitored and/or loggedvia appropriate hardware such as an external computer 250 and/or a fuelcell electrical control unit (ECU) 230. Various functions relating tooptimization of the fuel cell system operation may be incorporated intothe ECU, while an external computer may be used for detailed diagnosticsand testing of the fuel cell system via information made available onthe CAN.

The cell voltage profile data may be used to improve the efficiency andperformance of the fuel cell over time and under different conditions,through comparison with known profiles. For example, a distribution ofcell voltages across the fuel cell stack being lower at the edges of thestack 110 and rising in the center of the stack 110 typically indicatesthat the fuel cell stack 110 is cold or is receiving too much cooling.The reverse situation, i.e. where the voltage levels fall towards thecenter of the stack 110, indicates that the fuel cell stack 110 is hotor is receiving too little cooling. The former situation can be remediedby decreasing the level of cooling and/or applying heat to the ends ofthe stack 110, while the latter situation can be remedied by increasingthe level of cooling and/or reducing a level of heat applied to the endsof the stack 110. The ECU may be configured to monitor the voltagelevels of the fuel cell stack 110 at set time intervals, typically every100 ms. For diagnosis and optimization of fuel cell behavior, monitoringof the voltage levels may also be carried out over longer timeintervals, typically of the order of minutes or hours, and may be aimedat maximizing the lifetime of the fuel cell stack rather than optimizingits immediate operational efficiency.

The temperature of the cathode inlet and/or outlet is preferablymonitored by a fuel cell system controller, such as the ECU 230. Thismonitoring may include taking actual temperature measurements of thecathode inlet and/or outlet streams, for example by means of temperaturesensors TX2, TX3. The anode exit temperature may also be monitored, forexample by means of a temperature sensor on the anode exit line 165.Alternatively, or additionally, the temperature of the cathode inletand/or outlet may be monitored indirectly through measurements of otherfuel cell parameters in conjunction with a known predetermined thermalbehavior model of the fuel cell stack 110. The parameters may, forexample, be those of time and electric current drawn over time. Takinginto account the known thermal behavior of the fuel cell stack allowsthe fuel cell controller 230 to determine indirectly at what point thecathode flow path passing through the fuel cell stack 110 reaches theminimum required temperature for commencement of water injection. Thethermal behavior model may, for example, include parameters such as therate at which heat is lost to the surrounding environment for a range oftemperatures, and the heating effect in the fuel cell stack for a rangeof current drawn. By integrating a measure of current drawn over time,together with any additional heating effect due to components such asthe end plate heaters 330 a, 330 b, while taking into account heat lostfrom the stack 110 over this time, an estimate of the temperature withinthe cathode fluid flow path can be calculated.

In a general aspect therefore, monitoring the temperature of the cathodeinlet and/or outlet may comprise taking temperature measurements of thecathode inlet and/or exit streams. Monitoring the temperature of thecathode inlet and/or outlet may comprise calculating an estimate of thecathode inlet and/or exit streams using a measurement of current drawnfrom the fuel cell stack over time. The latter approach preferably takesinto account a predetermined thermal model of the fuel cell stack 110.

An additional use for the cell voltage information is through use of anoptimization algorithm which seeks to maximize fuel cell health andoverall system efficiency. The optimization algorithm should require noknowledge of the mechanisms of the system, and be arranged to provide asolution based on final values of the relevant criteria. In a simplifiedform, the optimization may seek to reduce, and preferably minimize, thefollowing cost function:

f(σ_(v) ,P _(p))=ασ_(v) ² +βP _(p) ²

where σ_(v) is a standard deviation of the voltage outputs of theplurality of cells, Pp is a parasitic load and σ, β are constants.Alternatively, the optimization algorithm may use the standard deviationof the cell voltage outputs alone, seeking to reduce or minimize this tooptimize the output of the stack 110.

The above cost is typically calculated at prescribed intervals by takinga snapshot of the fuel cell system data including the distribution ofcell voltages on voltage lines 220. Within a certain range, the standarddeviation of the individual cell voltages of the fuel cell stack aredependent upon the air stoichiometry of the system. In this context, thestoichiometry of the system refers to the molar quantity of oxygenavailable within the cathode volume of the fuel cell stack 110 comparedto the amount necessary to react with the amount of fuel being fed intothe anode volume. A stoichiometric balance of oxygen and hydrogen isindicated by the overall reaction:

2H₂+O₂→2H₂O

For a stoichiometric balance according to the above equation, twice asmany moles of hydrogen gas are required as moles of oxygen gas. Acathode stoichiometry of 2 therefore indicates that the same number ofmoles of oxygen 02 are passing through the cathode system as there aremoles of hydrogen H2 entering the anode system. Typically, astoichiometry of at least 2 is required to maintain reaction efficiencyin a normally-closed cathode system. In an open cathode system, thestoichiometry may be as high as 50, i.e. indicating that there are 25times as many moles of oxygen gas available as there are moles ofhydrogen gas. An increase in stoichiometry towards an oxygen richbalance typically results in an increase in gross fuel cell stackperformance and a reduction in fuel cell voltage standard deviation.However, in order to achieve this increase in available oxygen contentan increase in parasitic load due to the air delivery method used(typically the air compressor 133) is required. Hence, the cost functionabove is preferably balanced so as to achieve a suitable balance betweenparasitic load and fuel cell stack voltage output distribution.

The parasitic load on the fuel cell stack may be indicated by a measureof electric power consumed by one or more components of the fuel cellsystem 100 during operation. A measure of parasitic load can thereforebe determined from measurements of current provided to one or more ofthe: air compressor 133; heater plate(s) 330 (described in more detailbelow); and heaters 138, 145 for raising the temperature of the cathodeand anode inlet streams. A principle measure of parasitic load may beindicated by a measure of auxiliary electric power drawn by the aircompressor 133, since this controls the stoichiometric balance of gasesin the fuel cell stack 110. Such a measurement could, for example, beobtained through measuring current drawn by the air compressor 133and/or other electrically-operated devices such as pumps, valves,sensors, actuators and controllers. The air compressor 133 may bepowered from a high voltage supply, in which case a measurement ofcurrent drawn from this supply can provide the necessary indication.

Starting from an initial condition, an optimization routine typicallyupdates the cathode (air) stoichiometry set point at set time intervals,for example each minute. This allows the system to gradually optimizeaccording to different ambient conditions such as changes in ambientpressure (e.g. altitude) or temperature and stack health (e.g. stackdegradation due to aging).

To aid start-up of the fuel cell system 100 from sub-zero ambienttemperature, some or all of the following features may be required:

-   -   i) a heated hydrogen exit valve 161 (purge valve) and water        separator/collector 163 (shown in FIG. 1);    -   ii) heaters 138, 145 for raising the temperature of the cathode        and anode inlet streams;    -   iii) heaters 330 to raise the temperature of the current        collectors in the fuel cell, further detailed below with        reference to FIGS. 3, 4 a and 4 b;    -   iv) a source of liquid water available for introduction to the        fuel cell stack, such as the water containment vessel 140 (FIG.        1);    -   v) trace heating of lines for carrying liquid water, including        the water injection line 125 and water ejection line 128 (FIG.        1); and    -   vi) heating of an area around the fuel cell water injection        inlet 127.

An exemplary startup procedure may be detailed as follows. First, theair compressor 133 is started and set to provide a fixed flow rate tothe fuel cell stack cathode air inlet 126. For a fuel cell stack havinga 200 square cm active area, the flow required may be set to achieveaccording to a current set point of 80 A or more. This is followed byheating of the water lines 125, 128, the hydrogen exit valve 161, thefuel cell stack current collectors 320 a, 320 b (to be described inconnection with FIG. 3) and scavenge pump 132 between the waterseparator 131 and water containment vessel 140. Heaters on the cathodeand anode inlet lines 123, 155 are activated such that the inlettemperatures of the gas streams at the cathode air inlet 126 and anodefuel inlet 156 are preferably between 5 and 10° C. When starting fromsub-zero ambient conditions, the temperature of each stream is regulatedto a maximum of 10° C. in order to ensure that any water at the top ofthe fuel cell stack (where the gases are typically introduced) does notdefrost too quickly and subsequently freeze in the lower section of thefuel cell stack, which may still be below freezing point. The gases arethereby used at least partly to heat the cathode and anode fluidpathways to a degree such that the fuel cell stack does not cause waterinjected into the stack via the water injection inlet 127 to freeze.

The anode inlet valve 153 and purge valve 161 are then activated. Inthis startup state, an aggressive action for the purge valve isinstigated by repeatedly activating the purge valve 161 to promoteself-heating within the valve 161 to defrost and to dislodge, throughvibrations caused by the repeated activation, any small build up of icethat might prevent immediate opening of the valve.

Until the cathode inlet and exit temperatures are above at least 5° C.,the fuel cell stack is operated without cooling/humidification viacathode water injection. This is to ensure that introduction of waterthrough the cathode water injection inlet does not result in iceformation within the cathode volume of the fuel cell stack 110.

The fuel cell ECU takes control of the current that is drawn from thestack 110. An upper limit is set for the current that may be drawn, andthe fuel cell ECU then dictates what current should be drawn from thefuel cell. This current limit is between zero and the upper currentlimit, and is set by the ECU. This current limit should be less than orequal to the rated current for the fuel cell. For a more rapid startup,the fuel cell ECU 230 can set the current drawn from the fuel cell stack110 as high as is permitted by the values set on digital lines 211, 212.The fuel cell ECU 230 continuously monitors the health of the fuel cellstack 110 and applies or removes the load 260 accordingly. The load 260is generally applied and removed at fixed rates with respect to time,usually such that the current is reduced, i.e. on occurrence of avoltage warning threshold being passed, at a rate higher than the rateat which the current is increased when no warning threshold is passed.The fuel cell current is increased such that the current increasesaccording to a target control line and until a rated current of the fuelcell stack 110 is reached. However, the fuel cell ECU 230 primarily usesthe warning level on line 212, i.e. the upper of the two voltagethreshold indicators, to regulate the current that is drawn from thefuel cell stack 110 if the voltage of one or more cells falls below thewarning (or second) voltage threshold. The basic premise is to keepincreasing the current drawn from the fuel cell in line with apredetermined target control line until a warning is indicated. Thepredetermined rate at which the current is increased may be setaccording to particular characteristics of the fuel cell stack such asthe stack size, and the rate may be predetermined to vary according to,for example, the magnitude of current drawn from the stack or a measureof temperature. The maximum rate at which the current is increased ispreferably a predetermined value, typically between 1 and 3 Amps persecond depending on the size of the stack. This maximum rate determinesthe fastest time that the fuel cell system can reach full output powerfrom a cold start. If a current set point request, e.g. received from anexternal system, is less than this maximum rate, the fuel cell systemwill follow this lower value. After a warning is indicated, the currentis then reduced until the warning disappears. Hence, the controlessentially applies the maximum current that the fuel cell can handlewithout triggering a cell warning. An advantage of this approach is thatheat generated by the fuel cell increases with an increased current,hence higher currents equate to a faster time to defrost. This processof initial heating preferably occurs before any injection ofcooling/humidification water.

In a general aspect, the current drawn from the fuel cell stack 110 islimited to prevent a sum of cell voltages across the fuel cell stackfalling below a third voltage threshold, the third voltage thresholdbeing higher than the second (warning) voltage threshold multiplied bythe number of cells in the fuel cell stack 110. However, if the voltageof any individual cell falls below the warning voltage threshold, thecurrent is limited until the voltage rises again above the threshold.

Starting from cold, the total voltage of the stack may be regulated at apreset constant value, this value being the number of cells multipliedby a preset regulated voltage for each cell. A typical voltage for anindividual cell may be around 0.65V, and therefore a regulated voltagefor a 20 cell stack will be 13V. Although the total stack voltage isregulated, if an individual cell falls below warning voltage threshold,for example 0.4V or around 62% of the rated voltage, the current drawnis further regulated the prevent the cell voltage from falling further.

A preset ramp rate increase for the current may be applied rather thanregulation using the overall stack voltage. However, the preset voltagecan be used to automatically correct for stack starting temperature andother conditions.

In practice, a mathematical function may be used as the set point forthe stack current, which could take into account one or more factorsincluding stack voltage, temperature of the stack, ambient temperature,time from start and a standard deviation of all the cell voltages.

Once the temperature of the cathode inlet 156 and exit 159 are above 5°C., external cooling/humidification water may be added to the fuel cellstack 110 via the cathode water injection inlet 127. Also at this point,the control of the fuel cell current may revert to some other methodwhich is utilized for normal operation, and any heaters on the fuel cellstack 110 may be switched off.

FIG. 3 illustrates schematically a side view of an exemplary fuel cellstack 110. The stack comprises a stack of individual fuel cells 310,with current collector plates 320 a, 320 b at opposing ends of the stackof cells 310. Heater plates 330 a, 330 b are provided towards opposingends of the fuel cell stack 110, each heater plate 330 a, 330 b beingdisposed between a respective current collector plate 320 a, 320 b and arespective end plate 350 a, 350 b. Each heater plate 320 a, 320 b isthermally and electrically insulated from the respective end plate 350a, 350 b, preferably by means of further insulator plates 340 a, 340 bdisposed between the respective heater plates 330 a, 330 b and endplates 350 a, 350 b.

The main objective of the heater plates 330 a, 330 b is to heat up theend cells 311 at the same rate as the rest of the cells in the middle ofthe stack of cells 310. The heater plates 330 also warm the water feedchannel to the manifolds so that when the water is switched on it doesnot freeze.

Each heater plate 330, as shown in FIGS. 4 a and 4 b, is constructedfrom two electrical heating circuits. The circuits, for example in theform of copper tracks, are preferably embedded in the plate 330 andthereby isolated from an adjacent current collector. FIG. 4 a shows aperspective view of one face of an exemplary heater plate 330, whileFIG. 4 b shows a perspective view of the opposing face of the sameheater plate 330. The plate 330 generally comprises two buried tracks inthe form of electrically conductive heating elements 410, 420 formed ona printed circuit board 430, the heating elements 410, 420 formingserpentine tracks running across the heater plate over an areacorresponding to the active area of the underlying fuel cells in thefuel cell stack 110. For clarity, the buried tracks 410, 420 are shownto be visible in FIG. 4 a, but in practice the tracks may not be visiblethrough being covered by an electrically insulating cover layer and/or afurther circuit board. The heater plate 330 is powered externally via anelectrical source such as a storage battery, with positive and negativeterminal connections via side tabs 411, 412, 413, 414 in the form ofspade connections on an edge of the heater plate 330. These tabs 411,412, 413, 414, although located close together for wiring convenience,are preferably separated by an air gap 415, 416. The air gaps 415, 416act to prevent condensed water, which may form during the thawingprocess, from causing an electrical short circuit.

In addition to the function of heating the current collector, the heaterplate 330 can also serve to transfer water injected into the fuel cellstack 110 (for cooling and humidification) from a single water injectioninlet 450 to ports 460 corresponding to multiple galleries that runalong the length of the fuel cell stack 110, the galleries beingconfigured to deliver water to each individual cell. Water distributiontracks 470 between the inlet 450 and the ports 460 are designed suchthat they are of substantially equal length, so that the pressure dropand consequent flow rate along each track is equal. The waterdistribution feature is only required for one of the heater plates 330a, 330 b, because water is typically injected at only one end of thefuel cell stack 110. Each heater plate 330 also contains further ports470 to allow air and hydrogen to pass through to the individual cells.

Further illustrated in FIG. 5 is a cross-sectional schematic view ofpart of the fuel cell stack 110 of FIG. 3. A water feed line 510 allowsentry of water through the end plate 350, water being directed along apath indicated by arrow 520. The water feed line 510 preferablycomprises a heating element to prevent freezing of water passing throughthe line 510. Water passes through the end plate, the insulation layer340, past the water injection inlet in the heater plate 330, along thewater distribution tracks 470, through the ports 460 (FIG. 4 b) andalong the distribution galleries for distribution to the individualcells 310. The current collector plate 320 transmits electric currentfrom the stack 110 through an attached cable 530 to the load 260 (FIG.2). A benefit of having water distribution tracks 470 in the end plate35 is that a separate water distribution plate is not required,therefore requiring one fewer component in the fuel cell stack. Afurther advantage is that the channels are pre-heated, which avoidswater freezing on entering the stack.

The construction of fuel cell stack 110 shown in FIG. 5 allows cells 311at opposing ends of the stack of cells 310 to heat up rapidly, throughthe heater plates 330 being insulated from the end plates 350. The endplates 350 will generally have a high thermal mass through the need toprovide a rigid support structure for even application of compressivepressure across the active area of each of the cells 310. This highthermal mass, if not thermally insulated from the cells 310, will tendto slow the rate of heating at the ends of the stack 110. The individualbipolar plates in the stack 110, however, can be constructed to have alower thermal mass and can consequently be heated rapidly during thestartup procedure. By insulating the end plates, the cells 310 cantherefore be heated up more rapidly, allowing for a shorter startup timefrom cold. Preferably, sufficient heat is applied through the heaterplates 330 so that the ends of the fuel cell stack 110 heat up at asimilar rate to the middle. Typically, the current collector heaters aresized such that they draw sufficient power when operated to heat the endcells of the stack during startup. If the power drawn is too low theheaters do not heat the cells up sufficiently during startup, and if thepower is too high the end cells will tend to overheat and thus limit theperformance of the stack.

During a typical start-up from an extended period at sub-zerotemperatures, all the components shown in FIGS. 4 and 5 would be belowzero Celsius. When the system is started, fuel and oxidant are suppliedto the fuel stack cells 310. Electric current then begins to be drawn,and the cells 310 begin to heat up. The heater plate 330 is activatedduring start-up such that the current pick off plate 320 heats up at asimilar rate to the cells 310, which will tend to have a lower thermalinertia as well as being more thermally insulated compared with the cellend plates 350. The cells 319 will eventually reach a temperature wherewater injection is required to prevent overheating. In a typical fuelcell this will be within a period of around 15 to 60 seconds, whenstarting from a temperature of −20° C. At this point, water is injectedvia the heated water feed line pipe 510 (also shown as the cathode waterinjection line 125 in FIG. 1). It is important that all of thepassageways from the pipe 510 through to the individual cells 310 areclear of ice at this point. The water is passed through the end plate350 and over the heater plate 340 to prevent the water from freezing inthe internal transfer ports and water distribution tracks 450, 460, 470(FIG. 4 b).

Advantages of the heater plates 330 as described above include one ormore of the following:

-   -   i) the plates 330 a, 330 b allow for rapid electrical heating of        the current collectors of the fuel cell stack 110;    -   ii) connection to an electrical supply is made such that short        circuits via condensing water droplets are prevented;    -   iii) even distribution of cooling water from a single injection        point to the appropriate distribution galleries is enabled by        the use of distribution tracks 470 of even length;    -   iv) anode and cathode input and exit fluids can pass through the        heater plates 330 a, 330 b;    -   v) a reduced thermal lag at the ends of the fuel cell stack        results in an improved balancing of the thermal profile of the        stack; and    -   vi) water can be injected earlier, to prevent cells in the        center of the stack 110 from overheating, than would otherwise        be the case without heater plates 330.

FIG. 6 illustrates a schematic flow diagram of an exemplary procedurefollowed during starting operation of a fuel cell system according tothe disclosure. The first step 610 is to start operation, for example byapplying electrical power (e.g. from a battery storage unit) to theelectrical control unit 230 (FIG. 2). The ECU then, at step 611,operates the anode inlet valve 153 (figure I), optionally operating anintegrated heater on the valve 153 and/or by activating the solenoid inthe valve, as described above. The ECU can determine whether the anodevalve is open (step 612) by, for example, monitoring the pressurereading on pressure sensor PX1 (FIG. 1) near or at the anode inlet 156.

Once the anode valve 153 is open, the air compressor 133 is activated(step 613). Alternatively, the air compressor 133 could be activatedprior to operating the anode valve 153. An initial current limit is thenset for the fuel cell stack 110, at step 614. This initial current limitcan be zero or a higher level at which the fuel cell stack 110 can beginsafe operation from cold.

During the period when the fuel cell stack 110 is warming up, the ECUproceeds to make decisions based on whether the minimum voltage outputfrom the cells in the stack, Vmin, is higher than the first and secondthreshold voltage levels V₁ and V₂, at steps 615 and 618. As describedabove, these decisions can be made on the basis of the values present ondigital lines 211, 212 (FIG. 2). If, at step 615, the minimum cellvoltage level is not greater than the first voltage threshold level V₁,the current output is shut off (step 616). The process then waits (step617) for a preset period, typically a few seconds, before reconnectingthe current. The current limit may then be set to the level it wasbefore it was shut off, or reset to the initial current limit. If theminimum voltage output is not less than V₁, but is not greater than V₂,the warning or second voltage threshold level, the current limit isreduced (step 619) until V_(min) is greater than V₂.

The current limit is then increased (step 620) by a preset amount. Therate at which the current limit is increased may be a set amount, suchas 0.5 Amps per second, or some other rate dependent on the presentlyset level.

At step 621, an assessment of whether the temperature readings in theinlet 123 and outlet 121 lines of the cathode flow path, T_(in), T_(out)respectively, are greater than a minimum required temperature, T_(min).These temperature readings can be obtained, for example, fromtemperature sensors TX2, TX3 (FIG. 1). If both temperature readings aregreater than T_(min), the water injection system is activated, at step622. Alternatively, the decision at step 621 may depend solely on thetemperature T_(out) of the cathode outlet line. The water injectionsystem then continues in operation, varied according to the temperatureof the cathode air stream, until or unless the temperature of the airstream falls below the minimum level T_(min) or if the fuel cell systemis to be shut down.

During start-up, an assessment is made, at step 623, of whether thecurrent limit I has reached the rated current of the fuel cell stack110. If the current limit is less than the rated current, I_(rated), thestart-up process continues, proceeding to the previous step 615. Oncethe current limit is reached, the fuel cell system proceeds to acontinuous mode of operation, at step 624.

During continuous operation, the fuel cell system 100 preferablycontinues monitoring the voltage level V_(min) and temperature ofvarious parts of the system 100. The ECU also continues to monitor theoperation and adapt the operating parameters of the system 100 tooptimize operation, as described above.

FIG. 7 illustrates exemplary data from a fuel cell system duringstartup, in which the load current 710 rises from zero up towards therated current, in this case 100 A. The stack voltage 720 varies as aresult during this rise in current. Also shown in FIG. 7 are curvescorresponding to variation in the cathode exhaust temperature 730, theend plate water control temperature 740, the end plate air temperature750, the anode exhaust temperature 760, the water pump back pressure 770and the cathode water flow rate 780 during start-up.

The test illustrated in FIG. 7 was carried out on a 20 cell stack. A setpoint of 13 volts was used for the ECU 230, operating in a closed loopcontrol mode. Initially, with the stack starting from cold (i.e. at −20°C.), a set point of 13V was achieved with a current load of a few amps.As the stack warms up. the ECU tries to regulate the stack voltage to13V and ramps tip the current 710. At the end of a first time period711, the stack voltage 720 falls due to one or more of the cellsperforming less well, in this case due to overheating. The ECU thenreduces the current as a result. At the end of a second time period 712,the water injection system is turned on. Once water is injected into thestack, the voltage rises. The ECU then ramps up the current 710 untilthe anode exhaust temperature passes 0° C. At this point the stack isconsidered to be thawed out, so the current 710 is ramped more rapidlyto the full load point of 100 A.

In the strategy described above, the ramp rates for increasing thecurrent load are limited to predetermined maximum levels. In theparticular test illustrated by FIG. 7, the water injection system wasactivated only when the cathode exhaust reached 20° C., in order toensure that water did not freeze in the stack on being injected.

In the test illustrated by FIG. 7, during the initial first time period711 the current load 710 gradually increases from zero to around 40 A.while the measured stack voltage 720 remains roughly constant (after aninitial fall on application of the load). After the ECU detects that thevoltage of one or more of the cells in the stack has fallen below awarning threshold level, the current load 710 is gradually reduced overthe second time period 712 until the warning voltage threshold isexceeded. Over the first and second time periods 711, 712, thetemperature of the cathode exhaust temperature 730 rises and, during thesecond time period 712, exceeds 20° C., at which point the waterinjection system is activated. The start of water injection is indicatedby a sudden increase in the cooling water flow rate 780, followed by asmall fall in the cathode exhaust temperature 730. The end plate waterand air temperature 740, 750 continue to gradually rise throughout thestart-up period, as the end plate heaters 330 are activated and thestack continues to warm.

During a third time period 713, the current load 710 continues to rise,though at a reduced rate limited by the voltage output of the cells ofthe stack. A sharp rise in the anode exhaust temperature 760 towards theend of this period 713 indicates that the cells in the stack areoptimally heated and humidified. This is followed by a faster rise incurrent load during a fourth time period 714, during which the currentload does not need to be backed off due to low cell voltage. The ratedcurrent of 100 A is then reached and the fuel cell system beginscontinuous operation over a fifth time period 715. At shutdown 716 ofthe fuel cell system, between 17 and 18 minutes after initial start-up,the current load 710 is cut and the water injection system disabled, thelatter indicated by a sharp drop in the water pump back pressure 770.The stack voltage 720 rapidly rises in the absence of the current load710, and then gradually falls off as the remaining fuel in the fuel cell110 dissipates.

While the method and agent have been described in terms of what arepresently considered to be the most practical and preferredimplementations, it is to be understood that the disclosure need not belimited to the disclosed embodiments. It is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the claims, the scope of which should be accorded the broadestinterpretation so as to encompass all such modifications and similarstructures. The present disclosure includes any and all implementationsof the following claims.

It should also be understood that a variety of changes may be madewithout departing from the essence of the disclosure. Such changes arealso implicitly included in the description. They still fall within thescope of this disclosure. It should be understood that this disclosureis intended to yield a patent covering numerous aspects bothindependently and as an overall system and in both method and apparatusmodes.

Further, each of the various elements of the disclosure and claims mayalso be achieved in a variety of manners. This disclosure should beunderstood to encompass each such variation, be it a variation of animplementation of any apparatus implementation, a method or processimplementation, or even merely a variation of any element of these.

Particularly, it should be understood that as the disclosure relates toelements of the implementation, the words for each element may beexpressed by equivalent apparatus terms or method terms—even if only thefunction or result is the same.

Such equivalent, broader, or even more generic terms should beconsidered to be encompassed in the description of each element oraction. Such terms can be substituted where desired to make explicit theimplicitly broad coverage to which this disclosure is entitled.

It should be understood that all actions may be expressed as a means fortaking that action or as an element which causes that action.

Similarly, each physical element disclosed should be understood toencompass a disclosure of the action which that physical elementfacilitates.

Any patents, publications, or other references mentioned in thisapplication for patent are hereby incorporated by reference. Inaddition, as to each term used it should be understood that unless itsutilization in this application is inconsistent with suchinterpretation, common dictionary definitions should be understood asincorporated for each term and all definitions, alternative terms, andsynonyms such as contained in at least one of a standard technicaldictionary recognized by artisans and the Random House Webster'sUnabridged Dictionary, latest edition are hereby incorporated byreference.

Finally, all referenced listed in the Information Disclosure Statementor other information statement filed with the application are herebyappended and hereby incorporated by reference; however, as to each ofthe above, to the extent that such information or statementsincorporated by reference might be considered inconsistent with thepatenting, such statements are expressly not to be considered as made bythe applicant(s).

In this regard it should be understood that for practical reasons and soas to avoid adding potentially hundreds of claims, the applicant haspresented claims with initial dependencies only.

Support should be understood to exist to the degree required under newmatter laws—including but not limited to United States Patent Law 35 USC132 or other such laws—to permit the addition of any of the variousdependencies or other elements presented under one independent claim orconcept as dependencies or elements under any other independent claim orconcept.

To the extent that insubstantial substitutes are made, to the extentthat the applicant did not in fact draft any claim so as to literallyencompass any particular embodiment, and to the extent otherwiseapplicable, the applicant should not be understood to have in any wayintended to or actually relinquished such coverage as the applicantsimply may not have been able to anticipate all eventualities; oneskilled in the art, should not be reasonably expected to have drafted aclaim that would have literally encompassed such alternatives.

Further, the use of the transitional phrase “comprising” is used tomaintain the “open-end” claims herein, according to traditional claiminterpretation. Thus, unless the context requires otherwise, it shouldbe understood that the term “compromise” or variations such as“comprises” or “comprising”, are intended to imply the inclusion of astated element or step or group of elements or steps but not theexclusion of any other element or step or group of elements or steps.

Such terms should be interpreted in their most expansive forms so as toafford the applicant the broadest coverage legally permissible.

All callouts associated with figures are hereby incorporated by thisreference.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description, as shown inthe accompanying drawing, shall be interpreted in an illustrative, andnot a limiting sense.

1. A fuel cell stack comprising a plurality of fuel cells, each end ofthe fuel cell stack having a heater plate disposed between a currentcollector plate and an end plate, each heater plate being thermallyinsulated from a respective end plate.
 2. The fuel cell stack of claim 1wherein each heater plate comprises a heating element in the form of anelectrically conductive track on the heater plate.
 3. The fuel cellstack of claim 2 wherein the electrically conductive track is in theform of a serpentine track across a portion of the heater platecorresponding to an active area of cells within the fuel cell stack. 4.The fuel cell stack of claim 2 wherein the electrically conductive trackis buried beneath a surface of the heater plate.
 5. The fuel cell stackof claim 2 wherein the heater plate comprises a pair of spade terminalsextending from an edge of the heater plate, the terminals beingseparated by an air gap.
 6. The fuel cell stack of claim 1 wherein theheater plate comprises a water distribution passageway configured toallow passage of coolant from a water injection line in communicationwith a first face of the heater plate through to one or more coolantports on an opposing second face of the heater plate.
 6. The fuel cellstack of claim 6 wherein the water injection line passes through the endplate to the first face of the heater plate.
 7. The fuel cell stack ofclaim 6 wherein the water distribution passageway is provided on thesecond face of the heater plate.